40 Questions
What is the primary means of metabolizing amino acid-derived nitrogen?
The sequential action of aminotransferases, glutamate dehydrogenase, and the urea cycle
Which of the following amino acids is non-essential?
Alanine
What is the precursor metabolite for the synthesis of glycine?
Pyruvate
Which of the following amino acids is essential and synthesized from pyruvate?
Valine
What is the role of glutamate dehydrogenase in amino acid metabolism?
To metabolize amino acid-derived nitrogen
Which of the following amino acids is synthesized from phenylalanine?
Tyrosine
What is the fate of amino acid-derived nitrogen in the body?
It is metabolized into urea
Which of the following amino acids is synthesized from methionine?
Cysteine
What is the result of transamination in terms of amino acid and α-Keto acid?
A new amino acid and a new α-Keto acid
What type of enzyme catalyzes transamination reactions?
Aminotransferases
What is the name of the enzyme that interconverts aspartate and α-Keto glutarate with glutamate and oxaloacetate?
Glutamate-oxaloacetate transaminase
What is the importance of transaminases that transfer amino groups to oxaloacetate?
They are a route for incorporating the – NH2 group into urea
What is the reactant amino acid in the reaction catalyzed by alanine transaminase?
Alanine
What is the product α-Keto acid in the reaction catalyzed by alanine transaminase?
Pyruvate
What is the role of transamination in the urea cycle?
It is a way to incorporate the – NH2 group into urea
What is the reactant α-Keto acid in the reaction catalyzed by aspartate transaminase?
α-Keto glutarate
What is the function of Pyrodoxal phosphate (PLP) in transaminases?
Cofactor for all Transaminases
Which enzyme is responsible for the oxidative deamination of glutamate?
Glutamate dehydrogenase
What is the byproduct of the oxidative deamination of glutamate?
NH3 and NADH
What is the role of NAD(P)+ in the oxidative deamination of glutamate?
Acts as a cofactor in the reaction
What is the ratio of reduced NADPH to oxidized NADP+ in the liver under normal conditions?
High
Which of the following is an activator of Glutamate dehydrogenase?
ADP
What is the function of Serine and threonine dehydratase?
Direct deamination of serine and threonine
What is the significance of the hydroxyl side chain of serine and threonine?
Facilitates direct deamination
What is the byproduct of the urea cycle that is less toxic than ammonia?
Urea
Where does the urea cycle occur mainly?
Liver
What is the reactant involved in the formation of citrulline in the urea cycle?
Ornithine
What is the source of the nitrogen atoms in urea?
NH3 and aspartate
What is the product of the reaction catalyzed by monoamine oxidase?
Ammonia
What is the substrate for D-amino acid oxidases?
D-amino acid
What is the role of N-acetylglutamate in the urea cycle?
Allosteric activator of carbamoyl phosphate synthase
How many ATP molecules are consumed in the overall stoichiometry of the urea cycle?
3
What is the byproduct of the reaction catalyzed by amino acid oxidases?
H2O2
Which enzyme is localized in the mitochondria?
Ornithine transcarbamylase
What is the consequence of missing or defective biogenesis of peroxisomes or L-amino acid oxidase?
Hyperaminoacidemia and hyperaciduria
What is the product of the reaction catalyzed by carbamoyl phosphate synthase?
Carbamoyl phosphate
What is the purpose of catalase in the reaction catalyzed by amino acid oxidases?
To break down H2O2
What is the source of ammonia for the synthesis of carbamoyl phosphate?
Glutamate dehydrogenase reaction
What is the function of L-amino acid oxidases?
To catalyze the deamination of lysine
What is the outcome of the nonenzymatic decarboxylation of keto acid by H2O2?
Formation of carboxylic acid with one less carbon atom
Study Notes
Amino Acids and Nitrogen Metabolism
- Essential amino acids cannot be synthesized in the body and must be obtained through the diet.
- Non-essential amino acids can be synthesized directly by transamination from metabolites readily available from major pathways.
- There are 10 essential amino acids: Histidine, Isoleucine, Leucine, Lysine, Methionine, Phenylalanine, Threonine, Tryptophan, and Valine.
- Some non-essential amino acids can be synthesized by special pathways: Ornithine, Arginine, Glycine, Proline, Serine, Cysteine, and Tyrosine.
Transamination
- Transamination is the transfer of an amino group from an amino acid to an α-keto acid to form a new amino acid and a new α-keto acid.
- Aminotransferases (transaminases) catalyze these reactions, such as Aspartate transaminase and Alanine transaminase.
- PLP (pyridoxal phosphate) is a cofactor of all transaminases.
Oxidative Deamination
- Oxidative deamination by glutamate dehydrogenase occurs in the mitochondrial matrix.
- Glutamate is converted to α-ketoglutarate, producing NADH and NH3.
Alternative Mechanisms for Deaminating Amino Acids
- Direct deamination by serine and threonine dehydratase: serine is converted to pyruvate and NH3, and threonine is converted to α-amino-β-ketobutyrate and NH3.
- Amino Acid Oxidases: L-amino acid oxidase and D-amino acid oxidase, which catalyze the deamination of amino acids, producing α-keto acids and NH3.
The Urea Cycle
- The urea cycle is a series of reactions that convert NH3 and aspartate into urea.
- The cycle starts with the reaction between ornithine and carbamoyl phosphate to form citrulline.
- The overall stoichiometry of the urea cycle is: CO2 + NH4 + 3 ATP + Aspartate + 2 H2O → Urea + 2 ADP + 2 Pi + AMP + PPi + Fumarate.
- The urea cycle occurs mainly in the liver and is important for removing excess ammonia from the body.
Enzymes of the Urea Cycle
- Carbamoyl phosphate synthase (Type I) catalyzes the reaction between CO2 and NH3 to form carbamoyl phosphate.
- Ornithine transcarbamylase, Argininosuccinate synthase, Argininosuccinase, and Arginase are the other enzymes involved in the urea cycle.
- These enzymes are localized in the mitochondria and cytosol.
This quiz covers the structure and functions of amino acids, including their role in metabolic pathways such as the urea cycle and gluconeogenesis. It also touches on the importance of essential amino acids.
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